1. Field of the Invention
The present invention relates to a television signal transmitter for use in a cable television system, or the like, in which an intermediate-frequency television signal modulated by an analog or digital picture signal is frequency-converted into a television signal on a channel for transmission.
2. Description of the Related Art
A conventional television signal transmitter is described below by using the circuit diagram shown in FIG. 3. An intermediate-frequency television signal is input to a mixer 23 via a first variable attenuator 21 and a preamplifier 22. The amount of attenuation in the variable attenuator 21 can be changed by an automatic gain control (AGC) voltage. A local oscillating signal is supplied from a local oscillator 24 to the mixer 23. The local oscillator 24 is constituted by a voltage-controlled oscillator including a varactor diode (not shown). A control voltage is applied to the varactor diode by a phase-locked loop (PLL) circuit 25. The local oscillator 24 oscillates at a frequency corresponding to the control voltage. Channel-setting data D for determining the local oscillating frequency is input to the PLL circuit 25.
The mixer 23 mixes the intermediate-frequency signal and the local oscillating frequency so that the intermediate-frequency signal is frequency-converted into a television signal on any channel in the range from the very high frequency (VHF) band to the ultrahigh-frequency (UHF) band. The obtained television signal is then transmitted. Thus, the channel used for transmission is directly determined by the local oscillating frequency. The local oscillating frequency is set by the channel-setting data D input to the PLL circuit 25.
The television signal output from the mixer 23 is amplified by a first-stage output amplifier 26, and is input to a bandpass filter 27. The bandpass filter 27 comprises one having a varactor diode (not shown), such as a double-tuned circuit. The passband center frequency of the bandpass filter 27, used as-a tuning frequency, is determined by the control voltage applied to the varactor diode by the PLL circuit 25. The television signal transmitter is designed so that the frequency of the television signal and the tuning frequency of the bandpass filter 27, which correspond to a single control voltage, can be made equal to each other.
After passing through the bandpass filter 27, the television signal is sequentially output via a second variable attenuator 28, a second-stage output amplifier 29, and a broadband bandpass filter 30. Also the amount of attenuation in the second variable attenuator 28 can be changed by an AGC voltage. In addition, the television signal is input from the second output amplifier 29 to a detector 31, and the AGC voltage output from the detector 31 is supplied to the first and second variable attenuators 21 and 28.
In the above-described construction, although the television signal output from the mixer 23 is a signal on any channel in the range from the VHF band to the UHF band, the covered range of channels outputable by a single television signal transmitter is limited to a predetermined frequency band. The frequency ratio of the highest frequency channel to the lowest frequency channel is set at two or less. Thus, the highest passband center frequency of the bandpass filter 27 is double the lowest passband center frequency or less. This avoids interference caused by the second harmonic of the television signal output from the mixer 23, and avoids interference caused by a leakage oscillating signal from the local oscillator 25 and by an image frequency signal.
As described above, the television signal transmitter is designed so that the frequency of the television signal output from the mixer 23 and the passband center frequency of the bandpass filter 27 can be made equal to each other, even if the control voltage applied to the local oscillator 24 and the bandpass filter 27 by the PLL circuit 25 is changed. However, actually, the sensitivities of changes in the two frequencies with respect to the control voltage differ, thus generating a difference between the two frequencies. This is because the oscillating frequency of the local oscillator 24 and the tuned frequency of the bandpass filter 27 differ by the frequency of the input intermediate-frequency signal. For example, as FIG. 4 shows, if the oscillating frequency of the local oscillator 25 and the tuned frequency of the bandpass filter 27 are set to predetermined values (reference values) when the control voltage is V1, the oscillating frequency changes by a frequency-change amount when the control voltage is changed to V2, which is caused by the occurrence of a so-called “tracking error”.
Consequently, a problem occurs in that the in-band characteristics of the television signal cannot be accurately transmitted since the passband center frequency of the bandpass filter 27 is shifted from the frequency of the television signal output from the mixer 23. Although the problem can be solved by broadening the pass band of the bandpass filter, interference caused by the leakage oscillating signal and the image frequency signal increases.